Broadband Excitation by Chirped Pulses: Application to Single Electron Spins in Diamond

Pulsed excitation of broad spectra requires very high field strengths if monochromatic pulses are used. If the corresponding high power is not available or not desirable, the pulses can be replaced by suitable low-power pulses that distribute the power over a wider bandwidth. As a simple case, we use microwave pulses with a linear frequency chirp. We use these pulses to excite spectra of single NV-centers in a Ramsey experiment. Compared to the conventional Ramsey experiment, our approach increases the bandwidth by at least an order of magnitude. Compared to the conventional ODMR experiment, the chirped Ramsey experiment does not suffer from power broadening and increases the resolution by at least an order of magnitude. As an additional benefit, the chirped Ramsey spectrum contains not only `allowed' single quantum transitions, but also `forbidden' zero- and double quantum transitions, which can be distinguished from the single quantum transitions by phase-shifting the readout pulse with respect to the excitation pulse or by variation of the external magnetic field strength.Comment: 13 pages, 7 figure

La valeur du yuan:Les paradoxes du taux de change d’équilibre

La sous-évaluation de la monnaie chinoise vis-à-vis du dollar est souvent dénoncée comme l’une des causes des déséquilibres mondiaux. Les autorités américaines soutiennent que la Chine manipule son taux de change tandis que la majorité des travaux empiriques estiment que le yuan est sous-évalué de 15 à 30 %. Cet article montre que les modèles de taux de change d’équilibre généralement utilisés ne sont pas pertinents pour une économie en développement telle que l’économie chinoise. La Chine souffre d’un sous-emploi massif ; un taux de change sous-évalué au regard des critères traditionnels lui permet d’impulser la croissance vigoureuse nécessaire pour résorber son chômage. Plus généralement, le taux de change optimal d’un pays émergent ne peut être défini que relativement à une stratégie de croissance : plus le pays désire une croissance vigoureuse, plus il est prêt à faire un effort d’épargne, plus il doit pratiquer un taux de change faible. La première partie de l’article décrit les points saillants de la croissance chinoise depuis 1994, où la Chine est passée en change fixe avec le dollar : croissance élevée mais sous-emploi massif, faible inflation, accumulation massive de réserves, forts taux d’épargne et d’investissement. La deuxième partie présente et critique les évaluations du taux de change d’équilibre du yuan selon différentes méthodes. La troisième développe une maquette où le taux de change d’un pays en développement est volontairement fixé à un bas niveau pour permettre une croissance vigoureuse, de sorte que sous-évalué du point de vue des critères usuels, il est optimal du point de vue de la stratégie de croissance ; enfin, la quatrième partie montre la difficulté du choix d’un régime de change pour la Chine

Electron spin resonance detected by a superconducting qubit

A new method for detecting the magnetic resonance of electronic spins at low temperature is demonstrated. It consists in measuring the signal emitted by the spins with a superconducting qubit that acts as a single-microwave-photon detector, resulting in an enhanced sensitivity. We implement this new type of electron-spin resonance spectroscopy using a hybrid quantum circuit in which a transmon qubit is coupled to a spin ensemble consisting of NV centers in diamond. With this setup we measure the NV center absorption spectrum at 30mK at an excitation level of \thicksim15\,\mu_{B} out of an ensemble of 10^{11} spins.Comment: 6 pages, 4 figures, submitted to PR

Electron-spin-resonance center of dangling bonds in undoped a-Si:H

A variety of electron-spin-resonance (ESR) spectra of dangling bond (g=2.0055) in undoped hydrogenated amorphous silicon (a-Si:H) have been measured by the echo-detected ESR of pulsed ESR as well as the usual continuous-wave (cw) ESR for a wide range of two experimental parameters of microwave frequency (ν=3–34 GHz) and 29Si content (p=1.6, 4.7, 9.1 at. %). Using those spectra, we have carried out spectral simulations on the whole dangling bond spectrum (a primary line and 29Si hf structure), and also have simulated ν and p dependence of the spectra. From detailed simulation analyses, we confirmed a previous identification of the dangling bond center by Stutzmann and Biegelsen [Phys. Rev. B 40, 9834 (1989)], and raised the reliability of ESR parameters; isotropic and anisotropic 29Si hyperfine interactions were determined to be approximately 7.4 and 2.1 mT, respectively, and g∥=2.0039, g⊥=2.0065. The ESR parameters indicate that the dangling bond center is localized predominantly on a single Si atom and is characterized as strongly p like, which are consistent with the case of the dangling bond at the interface between crystalline Si and SiO2, the Pb center

Electronic Structure of Band-Tail Electrons in a Si:H

Electronic structures of the light-induced electron spin resonance (LESR) centers in undoped a-Si:H have been investigated by means of pulsed ESR techniques. Overlapping LESR signals of g = 2.004 and 2.01 have been experimentally deconvoluted by using the difference in spin-lattice relaxation time between the two signals. The 29Si hyperfine structures of the 2.004 signal clearly show that the wave function of this center spreads mainly over two Si atoms, which suggests that the origin of g = 2.004 is electrons trapped at antibonding states of weak Si-Si bonds